The present invention relates to an optical fiber amplifier, and more particularly to an optical amplifier using an erbium-doped optical fiber.
In recent years, active research has been made on optical amplifiers which amplify the signal light without photoelectric conversion with a view to reducing the size and cost of repeaters for optical communication or compensating for losses due to light branching. The systems of optical amplification so far reported include one using semiconductor laser and another using an optical fiber whose core is doped with a rare-earth element such as erbium (Er). Many research and development attempts are being made on optical amplifiers using an Er-doped optical fiber (Er-doped optical fiber amplifiers) because of their advantages such as the high gain of 30 dB or more they provide in the 1.55 micron wavelength band, which is the lowest loss wavelength region for optical fibers, and the scarce polarization-dependence of the gain. For more information on one of these Er-doped optical fiber amplifiers, reference may be made to R. I. Laming et al., "Noise Characteristics of Erbium-Doped Fiber Amplifiers Pumped at 980 nm" in IEEE PHOTONICS TECHNOLOGY LETTERS, Vol. 2., No. 6, (June 1990) 418-421.
This Er-doped optical fiber amplifier amplifies a signal light by simultaneously bringing into incidence on an optical fiber a pumping light having a wavelength equal to the absorption wavelength of Er ions. Known incident directions of the pumping light are forward pumping to launch the signal light and the pumping light into incidence so that they propagate in the same direction in the optical fiber, backward pumping to launch the two lights so that they propagate in reverse directions to each other, and hybrid pumping using both forward pumping and backward pumping. The wavelength ranges of the pumping light in common use include the 0.5 micron, 0.6 micron, 0.8 micron, 0.98 micron and 1.48 micron bands, of which the 0.98 micron and 1.48 micron bands are considered the most useful for practical purposes as they are free from excited state absorption (a phenomenon in which excited electrons are excited to a still higher level) and can provide high gains.
An Er-doped optical fiber amplifier for use in optical communication should desirably give a high saturation output and be relatively noise-free (low in noise figure). In case of the 1.48 micron band pumping, however, the saturation output is high but the noise figure is high, while in case of the 0.98 micron band pumping, the saturation output is low but it provides a low noise figure.
The saturation output Ps can be represented by Equation (1) where Pp is the pumping light power, .lambda.s is the signal light wavelength and ;p is the pumping light wavelength: EQU Ps=(.lambda.p/.lambda.s)Pp (1)
Therefore, if .lambda.s equals 1.55 microns and a pumping light of 1.48 microns is used, the ratio of conversion of the pumping light into the signal light (Ps/Pp) will be 95%. Where a pumping light of 0.98 micron is used, Ps/Pp will be 63%.
Then the noise figure NF can be generally represented by EQU NF=2N2/(N2-N1) (2)
where N2 is the number of electrons at the upper level and N1, that at the lower level. In the case of 0.98 micron band pumping, the energy difference between the exciting level and the upper level will be sufficiently great, and N1 will be 0 under highly excited state condition, resulting in an ideal NF of 3 dB. However, in the 1.48 microns band pumping where the exciting level and the upper level are close to each other, electrons will remain at the exciting and the lower levels even if greatly the pumping light power is increased. Consequently, there will exist electrons of N1=0.38 N2, resulting in an NF=5.1 dB.